This application is based upon and claims the benefit of Japanese Patent Applications No. 11-48467 filed on Feb. 25, 1999, and No. 11-347682 filed on Dec. 17, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to gas sensor elements constituting various sensors such as oxygen sensors, NOx sensors, HC sensors, or CO sensors, which are used in an exhaust system of an automotive internal combustion engine while being exposed to exhaust gas including lead to control combustion of the internal combustion engine, and a method of producing the same.
2. Description of the Related Art
A xcex sensor element is conventionally used as such an oxygen sensor element for an oxygen sensor, which is disposed in an exhaust gas system of an automotive combustion engine to perform an air fuel ratio feedback control. As shown in FIG. 7, such xcex sensor element 9 has a solid electrolyte body 90 having oxygen ion conductivity, a reference gas side electrode 92, and a measurement gas side electrode 91. The reference gas side electrode 92 is provided to be exposed to atmosphere serving as reference gas, and the measurement gas side electrode 91 is provided to be exposed to exhaust gas as measurement gas.
The sensor element 9 further has a porous protective layer 93 formed by plasma spraying and covering the surface of the measurement gas side electrode 92. The porous protective layer 93 restricts a diffusion amount of exhaust gas appropriately to stabilize sensor output, and protects the measurement gas side electrode 91 from being physically and chemically corroded by exhaust gas. The porous protective layer 93 is, however, readily clogged with foreign matters contained in exhaust gas, thereby preventing diffusion of exhaust gas from the porous protective layer 93 excessively. As a result, responsibility and output of the gas sensor element may be lessened.
JP-A-8-50114 and JP-A-9-68515 propose countermeasures against the clogging of the porous protective layer. These countermeasure are effective to the clogging of the protective film by foreign matters such as components added to oil, which are diffused at a solid phase or a liquid phase. However, when exhaust gas includes lead, the conventional countermeasures are non-effective to lead, which is diffused at a gaseous phase. Lead having passed through the protective layer is adhered to the measurement gas side electrode and readily deteriorates sensor characteristics.
JP-A-9-113480 proposes a protective layer including a lead trap layer therein to prevent diffusion of lead to the measurement gas side electrode. However, the lead trap layer includes catalytic metal such as Pt, which also has a catalytic effect to exhaust gas. The lead trap layer absorbs exhaust gas components such as O2, HC, and CO for a long time period by the catalytic effect of the catalytic metal, thereby increasing time required for exhaust gas to pass through the protective layer and retarding exhaust gas from reaching the measurement gas side electrode. As a result, the responsibility of the gas sensor element is decreased.
Further, especially when the protective layer is exposed to lead containing exhaust gas having a temperature higher than 800xc2x0 C., the catalytic metal traps lead. The catalytic metal having trapped lead becomes to not absorb O2, but to absorbs only unburned components such as HC and CO. Accordingly, the unburned components spend a long time for passing through-the protective layer as compared to that of O2. Consequently, sensor output becomes unstable by being decreased only at a rich-gas side. This problem is not peculiar to the oxygen sensor, and occurs in NOx sensors, CO sensors, HC sensors, and the like similarly.
The present invention has been made in view of the above problems. An object of the present invention is to provide a gas sensor element usable in measurement gas including lead, without decreasing output and responsibility thereof, regardless of temperature of measurement gas.
According to the present invention, briefly, a gas sensor element has a porous protective layer covering a measurement gas side electrode. The porous protective layer includes a component as a lead getter, which reacts with lead contained in measurement gas. Accordingly, the protective layer can remove lead from measurement gas as a result of reaction between the lead getter and lead.
The measurement gas side electrode is not deteriorated by lead attached thereto. Since the lead getter does not have an absorbing property with respect to measurement gas, measurement gas can pass through the protective layer quickly. Therefore, the sensor responsibility and sensor output are not decreased regardless of temperature of measurement gas.
Preferably, the component as the lead getter in the protective layer reacts with lead at a temperature higher than approximately 500xc2x0 C. Accordingly, the protective layer can trap lead securely by reacting with lead, which is diffused at a gaseous phase. When the temperature of measurement gas is lower than 500xc2x0 C., lead is hardly diffused. Therefore, the measurement gas side electrode is not deteriorated by lead. More preferably, the lead getter can react with lead at a temperature up to approximately 1000xc2x0 C. This is because the temperature of measurement gas may be raised to 1000xc2x0 C.
Preferably, the lead getter is included in the protective layer as particles having an average particle diameter in a range of approximately 0.2 xcexcm to 2 xcexcm. When the average particle diameter is smaller than 0.2 xcexcm,the protective layer may have cracks therein when it is formed. When the average particle diameter is larger than 2 xcexcm, a surface area of the lead getter is decreased as a whole, resulting in low reactivity with lead. In this case, lead cannot be trapped by the lead getter securely.
Preferably, the lead getter is made of material selected from a group consisting of WO3, MoO3, La2O3, FE2O3, Nb2O5, which can readily react with lead an hardly have an absorbing property to measurement gas. Accordingly, the protective layer can trap lead securely while allowing measurement gas to pass through without delay. Consequently, the sensor outputand sensor responsibility are not decreased.
The protective layer can be formed on the measurement gas side electrode by spraying, dipping, or coating a source material including the component as the lead getter on the measurement gas side electrode. The source material may include metallic oxide particles in addition to lead getter particles. According to this method, a large amount of lead getter particles can be dispersed in the protective layer uniformly.